In current display systems employing spatial light modulators, light sources and color filters are used to produce primary colors for the display systems. The light source often comprises an arc lamp emitting light of white color. The color filter, such as a spinning color wheel having primary color segments separates the white color light from the light source into primary colors. The primary colors then sequentially illuminate pixels of the spatial light modulator so as to be modulated according to date of desired images.
The primary color light from such light source and color filter, especially when the color filter has only primary color segments, however, has many deficiencies in color and/or in optical efficiencies, such as poor brightness. The quality of the displayed images is thus degraded. Moreover, the light source, such as the arc lamp generally exhibits non-uniform illumination spectrum—that is the illumination intensity, as well as the perceived illumination intensity by viewer's eyes varies over the wavelength in the visible light range. This non-uniform spectrum, if passing through the color filter of primary color segments only may distort the colors derived from combinations of the primary colors from the color filter.
For correcting the deficiencies of the illumination light onto the spatial light modulator and the imbalance in the illumination light from the light source, number of attempts has been made. For example, the lengths of the color segments in the color filter can be made unequal according to the spectral properties of the color filter. Some color segments can be made longer than others so as to “boost” wavelengths that are deficient in the light from the light source. Many of these approaches, however, have disadvantages or are not efficient. For example, the whiteness of the white color generated from the primary colors after the light source and color filter is often shifted away from the desired whiteness. Specifically, the temperature of the white color is moved from where it should be. Some approaches have addressed this off-white problem and move the white color generated by the additional color segment towards the white color generated by the primary segments of the color filter, as illustrated in FIG. 1 and FIG. 2.
FIG. 1 is a typical color wheel in the art for use in digital display systems. The color wheel has primary color segments red (R), green (G), and blue (B); and a white boosting segment W. The W segment can be a clear (white) segment or has a color for compensating the white color produced by the R, G, and B segments. A way of compensating the color using the white boosting segment is demonstrated in FIG. 2. A primary color is such a color that at least one color in the color space of the displayed images can not be properly produced without the primary color. In other words, a primary color is a mandatory color to produce other colors of color images.
Referring to FIG. 2, the white color generate by the primary color segments (RGB in FIG. 1) is represented by White (RGB) in the Commission International d'Eclairage 1931 chromaticity chart (hereafter CIE chromaticity chart). For boosting the brightness (whiteness) of the generated white (RGB), the white boosting segment is added to the color filter. The white boosting segment has a color represented by the Neutral point that is generally away from the white (RGB). The combination color Wp of the white boosting segment and the primary color segments RGB lies between the white (RGB) and neutral and in the line connecting the two. This method moves the generated white color toward the white by RGB, thus decreasing the discrepancy between the white color generated by the white boosting segment and the white generated by the RGB. However, this method naturally decreases the control range of the white color for different light sources. That is, this method does not allow for the white color control in the full white range from hot white (e.g. of an arc lamp) to cold white.
Another disadvantage of the color wheel in the art is that when different light sources are used, modulation methods, such as the pulse-width-modulation algorithms will need to be modified, which certainly degrades compatibility of the color wheels to light sources.
Therefore, a method and apparatus for compensating the color of the illumination light with the impact on the brightness of the illumination light minimized are desired.